just create a new step in your quest order "RunLuaCode" and write into the empty field
CastSpellByName("NameOfSpell", onSelf);
But if you want to use a HoT before combat it would be a lot easier to integrate itinto your fight class.

Source:
http://wowprogramming.com/docs/api/CastPetAction
http://wowprogramming.com/docs/api/PetAttack
http://wowprogramming.com/docs/api/PetDismiss
http://wowprogramming.com/docs/api/PickupStablePet
all I did, was to google this ;)